This invention relates generally to magnetic flowmeters, and more particularly to a reference-voltage detecting circuit in a magnetic flowmeter whose electromagnet is excited by a low-frequency excitation current.
In a magnetic flowmeter, an electromagnetic field is established therein by an electromagnet having an excitation coil, the field being intercepted by a fluid passing through the flow tube to induce a flow rate signal in a pair of diametrically-opposed electrodes. Magnetic flowmeters are known which make use of a low-frequency excitation current, such as one whose excitation frequency is well below that of the commercial power-line frequency. One can, by means of a low-frequency excitation type magnetic flowmeter, obtain a flow rate signal having an excellent signal-to-noise ratio; for unwanted magnetic coupling and/or electrostatic coupling between the excitation coil and the signal lead wires connected to the flow tube electrodes may be reduced considerably.
In order to eliminate unwanted fluctuations from the flow rate signal yielded by a low-frequency excitation type magnetic flowmeter, which result from fluctuations in the excitation current for the electromagnet, the general practice is to provide a reference voltage-detecting circuit. This circuit is arranged to produce a reference voltage proportional to the excitation current, the ratio of this reference voltage to the flow rate signal being determined by means of a converter or divider.
To produce the reference voltage, it is known to interpose a resistor in the excitation circuit, the voltage drop across the resistor being proportional to the excitation current and therefore reflecting fluctuations in this current. The use of a resistor for this purpose has a number of disadvantages. For example, though it is desirable that the resistor have a low ohmic value in order to minimize the heat generated therein by reason of IR drop, in using a resistor of small value, the resultant reference voltage has a low level which requires amplification and adds to the cost and size of the instrument.
To overcome this drawback one could, in place of a resistor, make use of a current transformer in the excitation circuit to produce the required reference voltage. But since the frequency of the excitation current flowing through the transformer primary is very low, precise reference voltage detection cannot be effected by means of an ordinary current transformer of small size, and it becomes necessary, therefore, to employ a costly current transformer of very large size.